In particular, since not even input runs at real-time priority,
you can't stop it via any interactive means,
because the thread that manages input can't even run to process
your input.

Mind you, even if the input thread did run at real-time priority,
that wouldn't really help you any.
Sure, it could receive your input and distribute it to the appropriate
application queues,
but those applications are themselves not running with real-time
priority,
so all that happens is that your input gets quickly transferred to
the input queues, where it then sits waiting for the applications
to process them (which will never happen since the applications
are not running with high enough priority).

One might argue that Task Manager should run with real-time priority,
so it can extricate you from this situation, but that won't help,
and it would be wrong.
It won't help because you first need to be able to launch Task Manager
(or switch to it, if you were prescient enough to have it already
running),
and none of the ways of launching Task Manager run with real-time
priority.
(Because nothing in the user interface runs with real-time
priority.)

Second, even if there were a special code path that enabled you
to launch Task Manager
at real-time priority,
it would be wrong,
because the fact that Task Manager is running with real-time
priority means that it is now stealing CPU cycles from that
other process which set itself to real-time priority,
which defeats the purpose of that process setting itself
to real-time priority in the first place:
It set itself to real-time priority because it didn't want
anybody stealing CPU time from it!

What could be done is to have Task Manager display an extra
warning dialog when somebody uses it to change a process's priority.
The warning dialog would say something like
"Fiddling with priority can result in
you being totally screwed.
Are you sure you want to take this risk?"
(Oh wait, that dialog box already exists.)
Our friend Igor probably clicked right on past that warning dialog,
because he's thinking,
"Of course I want to set it to real-time priority,
you stupid program,
that's why I clicked Real-Time Priority!
This is another in a long string of examples of how
Windows is one of those coddling
operating systems that gets in the way of advanced users
like me, throwing up frustrating obstacles which
prevent me from getting things done."

And then he shoots himself in the foot and asks to be coddled a little bit.